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United States Department Mechanisms of Range Expansion of Agriculture Forest Service and Removal of Mesquite in Desert Rocky Mountain Grasslands of the Southwestern Research Station General Technical United States Report RMRS-GTR-81 October 2001 Thomas B. Wilson Robert H. Webb Thomas L. Thompson Abstract ___________________________________________ Wilson, Thomas B.; Webb, Robert H.; Thompson, Thomas L. 2001. Mechanisms of range expansion and removal of mesquite in desert grasslands of the Southwestern United States. Gen. Tech. Rep. RMRS-GTR-81. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 23 p. During the last 150 years, two species of mesquite trees in the Southwestern United States have become increasingly common in what formerly was desert grassland. These trees have spread from nearby watercourses onto relatively xeric upland areas, decreasing rangeland grass production. Management attempts to limit or reverse this spread have been largely unsuccessful. This paper reviews studies regarding mesquite natural history and management strategies, emphasizing studies published during the past decade. Mesquite possess a deep root system and are capable of fixing atmospheric N, rendering them capable of accessing resources unavailable to other plants in open rangeland. Their seeds, which remain viable for decades, have a hard exocarp and require scarification before germination. Consumption by cattle provides a means of scarification and seed dispersal, and is a major factor contributing to the spread of mesquite in open rangelands. Increases in atmospheric CO2 and winter precipitation during the past century also contribute to enhanced seed germination. Removal techniques have included herbicides, prescribed burning, grazing reduction, and mechanical removal. For increased effectiveness of these techniques, management goals must be clearly articulated; these goals include complete removal, no removal, and limited removal. Of these, limited removal appears the most feasible, using an initial herbicide application followed by periodic prescribed burning. Keywords: mesquite, biological invasions, rangeland ecology, soil nitrogen, rangeland management The Authors ________________________________________ Thomas B. Wilson, Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721. Robert H. Webb, U.S. Geological Survey, 1675 W. Anklam Road, Tucson, AZ 85745. Thomas L. Thompson, Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721. Acknowledgments ___________________________________ The funding for this project was provided by the Southwestern Borderlands Ecosystem Research Program of the USDA Forest Service, Rocky Mountain Research Station. The authors particularly thank Gerry Gottfried for his support of this project. Sheridan Stone of Fort Huachuca provided considerable support for our on-base work. Janice E. Bowers and Peter G. Griffiths of the U.S. Geological Survey and Thomas Biggs of the University of Virginia provided data for this report. John H. Brock, Peter F. Ffolliott, Gerald J. Gottfried, and Lorne G. Wilson critically reviewed the manuscript. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service Table of Contents Page Page Mesquite Management Practices ...................................... 17 Introduction ......................................................................... 1 Prescribed Burning ........................................................ 17 Mesquite Natural History ..................................................... 1 Herbicide Application ..................................................... 18 Floristics .......................................................................... 1 Mechanical Removal ..................................................... 19 Morphology ...................................................................... 8 Mesquite Management Objectives .................................... 19 Phenology and Germination ............................................ 8 Objective 1: Complete Mesquite Removal Mesquite and Nitrogen .................................................... 9 From Desert Grassland. ........................................... 19 Historical Range Expansion of Mesquite .......................... 10 Objective 2: Continued Mesquite Mechanisms of Mesquite Range Expansion ..................... 11 Establishment in Desert Grassland. ......................... 20 Fire ................................................................................ 11 Objective 3: Limited Mesquite Removal Seed Dispersal .............................................................. 12 From Desert Grassland. ........................................... 20 Livestock Grazing .......................................................... 14 Conclusions ...................................................................... 20 Climate Fluctuations and Atmospheric Gases .............. 14 References ........................................................................ 21 You may order additional copies of this publication by sending your mailing information in label form through one of the following media. Please specify the publication title and number. Telephone (970) 498-1392 FAX (970) 498-1396 E-mail [email protected] Web site http://www.fs.fed.us/rm Mailing Address Publications Distribution Rocky Mountain Research Station 240 West Prospect Road Fort Collins, CO 80526 Pesticide Precautionary Statement This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been registered. All uses of pesticides must be registered by appropriate State and/or Federal agencies before they can be recommended. CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlife—if they are not handled or applied properly. Use all pesticides selectively and carefully. Follow recommended practices for the disposal of surplus pesticides and pesticide containers. CAUTION: PESTICIDES Mechanisms of Range Expansion and Removal of Mesquite in Desert Grasslands of the Southwestern United States Thomas B. Wilson Robert H. Webb Thomas L. Thompson Introduction ____________________ of its natural history. Though this has been studied in the past, many studies concerning intraspecific com- Rangelands in the Southwestern United States petition (Ansley and others 1998; Kramp and others have been used for settlement, agriculture, and live- 1998), seed germination requirements (Bush and stock ranching during the last 150 years. Because of Van Auken 1990; Cox and others 1993), root systems the gentle topography, sufficient available water, and (Ansley and others 1990, 1991; Stromberg 1993), and abundant forage, use of this rangeland—especially in nitrogen fixation (Johnson and Mayeux 1990; Zitzer southeastern Arizona and southwestern New Mexico and others 1996) have taken place only within the last (fig. 1)—has been intensive. As a result, much of this 10 years. area has been transformed to a mixed-phase wood- In this study, we especially refer to the desert grass- land or shrubland, with no sign of this trend di- lands of southeastern Arizona and southwestern New minishing (Archer and others 1988; Buffington and Mexico in the area shown in figure 1. Previous workers Herbel 1965; Hastings and Turner 1965; McClaran (Brown and Lowe 1980) referred to two types of grass- and Van Devender 1995). Figures 2–6 show repeat land in this region—semidesert grassland and plains photography that document this change at selected and Great Basin Desert grassland—but we prefer the sites in the region. The alteration of grassland to simpler classification of “desert grassland” as used by shrubland has created considerable concern as land McClaran and Van Devender (1995). We subdivide the use managers recognize a corresponding decline in desert grasslands into the desert grasslands of south- available livestock forage. The mesquite tree (Prosopis eastern Arizona, which mostly surround the Santa spp.) has played a major role in this change, as it has Rita and Huachuca Mountains, and the desert grass- expanded its range from more sheltered and mesic lands of the western Chihuahuan Desert, which spans desert grassland drainage systems and riparian zones the area from the Sulfur Springs Valley of southeast- to open rangelands. ern Arizona to the Bootheel of southwestern New While the causes that account for this altered distri- Mexico. bution—climate change (Grover and Music 1990), The objectives of this study are (1) to describe the livestock grazing (Archer and others 1988; Bahr and natural history of the mesquite species that have Shelton 1993; Schlesinger and others 1990), suppres- increased their distribution within the Southwestern sion of wildfires (Bahre 1985; Biggs 1997; Humphrey United States, focusing on those attributes that could 1958), or rodent effects (Cox and others 1993; Reynolds account for this range expansion, (2) to review the and Glendening 1949)—have been investigated for existing research that has addressed this expansion, several decades, no single effective strategy has been with an emphasis on research produced within the last offered as a means of slowing or reversing mesquite decade, (3) to describe management goals, practices, encroachment in desert

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